I'd put in a grease zerk and put the shaft male-side up, and double zip-tie a mountain bike inner tube section to it near the u-joints. That'll keep the grease in and the dirt out. Be sure to take it off and clean it first though. 
In theory, its a good idea, but I've seen people put those zerks in, and then bust them off on their first run
Never thought about the inner tube though, thats a good idea. When I built mine, I actually put some duct tape around the male shaft, mostly to fill the little gap for slop.
might help do the same thing as the inner tube though, can't say if the duct tape is even still in there, I haven't torn it apart since I put it in.
